Method and apparatus for performing an automatic power adjustment for an optical signal

ABSTRACT

A method and apparatus for performing an automatic power adjustment wherein a signal power level of an optical signal transmitted by an optical transceiver via an optical span to a far-end device is adjusted automatically in response to a determined span loss of the optical span to achieve a predetermined desired receive signal power level of the optical signal at the far-end device.

TECHNICAL BACKGROUND

The invention relates to a method and apparatus for performing anautomatic power adjustment for an optical signal and in particular of anoptical signal within a wavelength division multiplexing WDM opticalnetwork.

In wavelength division multiplexing WDM networks, multiple opticalcarrier signals are multiplexed on a single optical fiber by usingdifferent wavelengths (also called colours) of laser light to carrydifferent signals. This increases data transmission capacity and enablesbi-directional communication over the same strand of optical fibers. Inwavelength divisional multiplexing optical networks, in particular indense wavelength division multiplexing DWDM optical networks, it isdesirable to attain a flat optical spectrum at the egress of each nodeof the network across all wavelengths. When a wavelength divisionalmultiplexing (WDM) optical network comprises fixed optical add/dropmultiplexing (FOADM) nodes, balancing of optical powers for allwavelengths can be challenging. These FOADM nodes comprise fixed OADMfilters that provide no power adjustment for power balancing. However,the transmitted optical power levels for the transponders and muxpondersinstalled in such FOADM nodes can vary up to 10 dB, i.e. the transmitpowers can vary from e.g. −3 dB to +7 dB. These power variations can befurther increased by the different loss of the different filters used bythe wavelengths on the multiplexing path.

Accordingly, in conventional wavelength divisional multiplexing (WDM)optical networks, balancing the optical spectrum for an optical add/dropmultiplexing node typically requires a manual power adjustment by a userusing fixed in-line signal attenuators and power measuring devices suchas power meters and/or spectrum analyzers. This procedure of manualbalancing the optical power levels is very cumbersome and time consumingand affects all users of the optical platform including customers,installers, and customer service engineers. Therefore, there is a needto provide a more manageable solution for balancing the optical powerlevels which avoid a manual power adjustment to balance the opticalspectrum at an optical add/drop multiplexing node of an optical network.

SUMMARY OF THE INVENTION

The invention provides a method for performing an automatic poweradjustment, wherein a signal power level of an optical signaltransmitted by an optical transceiver via at least one optical span to afar-end device is adjusted automatically in response to a determinedspan loss to achieve a predetermined receive signal power level of theoptical signal at the far-end device.

An optical span can be formed in a possible embodiment by a pair offiber jumpers between devices at one location or the optical span canalso be a pair of fibers between nodes at two different locations over adistance of several kilometers or miles.

In a possible embodiment of the method according to the presentinvention, at least one optical pilot signal is transmitted by saidoptical transceiver via a first fiber of said optical span towards thefar-end device and looped back via a second fiber of said optical spanto said optical transceiver which measures the signal power level of thelooped back optical pilot signal received by said optical transceiver.

In a possible embodiment of the method according to the presentinvention, the span loss of said optical span is determined on the basisof a transmit signal power level of a transmitted optical pilot signaland on the basis of the measured received signal power level of thereceived looped back optical pilot signal.

In a possible embodiment of the method according to the presentinvention, the optical pilot signal is looped back by a signal loop backunit being formed by loop adapter connected to said optical span at theside of the far-end device or being plugged into the far-end device.

In a possible embodiment of the method according to the presentinvention, the span loss is determined by comparing a measured signalpower level of an optical data signal received by the opticaltransceiver via a fiber of the optical span from said far-end devicewith a predetermined desired receive signal power level.

In a possible embodiment, the far-end device is an optical transceiverand at least one communication channel is established via said opticalspan between both optical transceivers.

The optical transceivers can be pluggable devices which can be pluggedin the near-end device and the far-end device.

In a possible embodiment of the method according to the presentinvention, each of the optical transceivers performs steps of:

measuring the signal power level of an optical data signal received bythe optical transceiver from the other optical transceiver via theoptical span,

comparing the measured signal power level with a predetermined desiredreceive power level to calculate a difference between the measuredreceive power level and the desired receive power level, and

transmitting a power adjustment message via the establishedcommunication channel to the other optical transceiver to adjust anattenuation or amplification of the signal power of the optical signaltransmitted by the optical transceiver via the optical span to the otheroptical transceiver.

In a possible embodiment, the established communication channel isformed by an embedded communication channel.

In an alternative embodiment, the established communication channel isformed by an out of band communication channel.

In a possible embodiment of the method according to the presentinvention, the power adjustment message is transmitted periodically bythe optical transceiver via the established communication channel to theother optical transceiver.

In a possible embodiment of the method according to the presentinvention, the signal power level of the optical signal is adjustedautomatically such that it is balanced with signal power levels of otheroptical signals transmitted over the same optical span at differentwavelengths.

In a possible embodiment of the method according to the presentinvention, the signal power level of the optical signal transmitted bythe optical transceiver is adjusted automatically in real time tocompensate changes of the transmission characteristics of the opticalspan.

The invention further provides an optical transceiver comprising atleast one signal adjustment unit to adjust automatically a signal powerlevel of an optical signal transmitted by said optical transceiver viaat least one optical span to a far-end device in response to adetermined span loss of said optical span to achieve a predetermineddesired receive signal power level of the optical signal received bysaid far-end device.

The signal adjustment unit can be formed by a variable opticalattenuator or a variable gain optical amplifier.

In a possible embodiment of the optical transceiver according to thepresent invention, the optical transceiver is a pluggable opticaltransceiver. This pluggable transceiver can be plugged into a near enddevice.

The near end device in which the pluggable optical transceiver isplugged into can comprise a transponder for a single client device or amuxponder for multiple client devices.

In a possible embodiment of the optical transceiver according to thepresent invention, the optical transceiver comprises a monitoring unitwhich is provided for monitoring at least one optical pilot signaltransmitted by said optical transceiver via a first fiber of saidoptical span towards the far-end device and looped back via a secondfiber of said optical span to said optical transceiver.

In the embodiment of the optical transceiver according to the presentinvention, the monitoring unit measures the received signal power levelof the looped back optical pilot signal and determines the span loss ofsaid optical span on the basis of the transmitted signal power of thetransmitted optical pilot signal and on the basis of the measuredreceive signal power level of the received looped back optical pilotsignal.

In an alternative embodiment of the optical transceiver according to thepresent invention, the optical transceiver comprises a monitoring unitwhich is provided to determine a span loss of said optical span bycomparing a measured signal power level of an optical signal received bysaid optical transceiver via a fiber of said optical span from thefar-end device with a predetermined desired receive signal power level.

In a possible embodiment of the optical transceiver according to thepresent invention, the far-end device comprises at least onemultichannel multiplexing/demultiplexing unit comprising a predeterminednumber of channel ports each being connectable to an opticaltransceiver.

In a further embodiment of the optical transceiver according to thepresent invention, the optical transceiver comprises an integratedcontrol unit generating a control signal to control the signaladjustment unit automatically in response to the determined span loss ofsaid optical span.

In an alternative embodiment of the optical transceiver, the opticaltransceiver comprises an interface for receiving a control signal tocontrol the signal adjustment unit automatically in response to thedetermined span loss of the optical span, wherein the received controlsignal is provided by a near end device to which the optical transceiveris directed connected.

The optical transceiver according to the present invention can be apluggable optical transceiver which can be plugged into a transponder ora muxponder of a node in an optical network, in particular an opticalWDM network. This node can comprise a multichannelmultiplexing/demultiplexing unit having a predetermined number ofchannel ports each being connectable to an optical transceiver,according to the present invention, plugged into a transponder or amuxponder of the respective node. The transponder can be provided for asingle client device such as a router or a switch and can be connectedto an optical transceiver, according to the present invention, pluggedinto the respective client device and connected to a correspondingtransceiver plugged into the transponder of the node. Each muxponder ofthe node can be provided for multiple client devices such as routers,switches etc. The node can be connected via an optical network tofurther nodes. In a possible embodiment, the node is connected to othernodes of the network in a ring structure. The node can be connected toother nodes in a network of any topology including linear, ring and meshnetworks.

Accordingly, the present invention further provides an add/dropmultiplexing node for an optical network comprising at least one networkinterface connected to a multichannel multiplexing/demultiplexing unitcomprising a predetermined number of channel ports each being connectedvia an optical span to an optical transceiver having a signal adjustmentunit to adjust automatically a signal power level of an optical signaltransmitted by said optical transceiver towards the respective channelport to achieve a predetermined desired receive signal power level ofthe optical signal.

In the embodiment of the add/drop multiplexing node according to thepresent invention, the multichannel multiplexing/demultiplexing unitcomprises a loop back unit to loop an optical pilot signal transmittedby said optical transceiver via a first fiber of said optical span tosaid channel port of said multiplexing/demultiplexing unit back to saidoptical transceiver via a second fiber of said optical span, wherein theoptical transceiver determines the span loss of said optical span on thebasis of the transmit signal power level of the transmitted opticalpilot signal and on the basis of the looped back optical pilot signal.

In a possible embodiment of the add/drop multiplexing node according tothe present invention, the node comprises a network west interface and anetwork east interface to connect said node to an optical network ring,wherein each network interface is connected to a multichannelmultiplexing/demultiplexing unit having a predetermined number ofchannel ports.

The add/drop multiplexing node can also comprise more than two networkinterfaces, for example within a mesh network.

In a possible embodiment of the add/drop multiplexing node according tothe present invention, the channel port of a multichannelmultiplexing/demultiplexing unit is either connectable to a channel portof the other multichannel multiplexing/demultiplexing unit to form apass through channel or to a transponder of a single client device or toa muxponder for multiple client devices.

BRIEF DESCRIPTION OF THE ENCLOSED FIGURES

In the following, possible embodiments of the method and apparatus forperforming an automatic power adjustment are described with reference tothe enclosed figures.

FIG. 1A, 1B show diagrams to illustrate a method and apparatusperforming an automatic power adjustment in a single ended operationaccording to a possible embodiment of the present invention;

FIG. 2 shows a diagram to illustrate a method and apparatus forperforming an automatic power adjustment in a dual-ended operationaccording to a further possible embodiment of the present invention;

FIG. 3 shows a diagram to illustrate an apparatus and a method forperforming an automatic power adjustment wherein one of the transceiversis provided in a client-connected device;

FIG. 4 shows an exemplary embodiment of an optical network comprisingnodes each employing a method and apparatus for performing an automaticpower adjustment according to the present invention;

FIG. 5 shows a block diagram of a possible embodiment of a node withinan optical ring network as shown in FIG. 4 employing the method andapparatus for performing an automatic power adjustment according to thepresent invention;

FIG. 6 illustrates a dual-ended operation for performing an automaticpower adjustment according to the present invention over an opticalnetwork;

FIG. 7 shows a block diagram of a possible embodiment of an opticaltransceiver according to the present invention;

FIG. 8 shows a block diagram of a further possible embodiment of anoptical transceiver according to the present invention;

FIG. 9 shows a block diagram of a further possible embodiment of thetransceiver according to the present invention;

FIG. 10 shows a diagram to illustrate a process of equalizing a non-flatoptical spectrum achieved by apparatus and a method for performing anautomatic power adjustment according to the present invention.

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS

As can be seen from FIGS. 1A, 1B, an optical transceiver 1 according tothe present invention can, in a possible embodiment, be plugged into anear end-device 2. In the embodiment shown in FIGS. 1A, 1B a looped backoptical signal is used for power adjustment. The near end-device 2 canbe, for example, a transponder for a single client device or a muxponderfor multiple client devices. The near-end device 2 can for example bealso a router, a switch or any other device that can house the opticaltransceiver 1. Such a transponder or muxponder 2 can form part of anadd/drop multiplexing node within an optical network. The pluggableoptical transceiver 1 shown in FIG. 1A is connected via an optical span4 of a far-end device 3. The optical span 4 can comprise optical fibersfor transmitting an optical signal. In the exemplary embodiment shown inFIG. 1A, the far-end device 3 is a passive device with a built-in orintegrated unit which can loop back a certain percentage of the opticalsignal received by the far-end device 3 from the optical transceiver 1via the optical span 4.

In a possible embodiment, the optical span 4 comprises two opticalfibers 4-1, 4-2 and the integrated loop back unit 5 loops a certainpercentage of the optical signal received by the far-end device 3 overthe first optical fiber 4-1 via the second optical fiber 4-2 back to theoptical transceiver 1.

The optical transceiver 1, according to the present invention, comprisesa signal adjustment unit which is adapted to adjust automatically asignal power level of the optical signal transmitted by the opticaltransceiver 1 via the optical span 4 to the far-end device 3 in responseto a determined span loss of the optical span to achieve a predetermineddesired receive signal power level of the optical signal received by thefar-end device 3. In a possible embodiment, the signal adjustment unitintegrated in the optical transceiver 1 is a variable opticalattenuator. In an alternative embodiment, the signal adjustment unitintegrated in the optical transceiver 1 is a variable gain opticalamplifier. In the embodiment of FIG. 1A, the loop back unit 5 isintegrated in the far-end device 3.

In an alternative embodiment as shown in FIG. 1B, the loop back unit isintegrated in a separate loop back adapter 6. The loop back adapter 6can be connected in the vicinity of the far-end device 3 to the opticalspan and can comprise an interface of a standard transceiver 7 of thefar-end device 3. The far-end device 3 can also be formed by atransponder or muxponder for one or several client devices.

In a preferred embodiment, the optical transceiver 1 transmits at leastone optical pilot signal via the first fiber 4-1 of said optical span 4towards the far-end device 3 wherein the optical pilot signal is loopedback via the second fiber 4-2 of said optical span 4 to the opticaltransceiver 1. The optical transceiver 1 measures the signal power levelof the looped back optical pilot signal received by the opticaltransceiver 1. The span loss of the optical span 4 is determined by theoptical transceiver 1 on the basis of the transmit signal power level ofthe transmitted optical pilot signal transmitted via the first fiber 4-1and on the basis of the measured receive signal power level of thereceived looped back optical pilot signal received via the second fiber4-2. The optical pilot signal is looped back by the signal loop backunit which is either formed by a loop adapter 6 as shown in FIG. 1Bconnected to the optical span 4 at the side of the far-end device 3 orbeing plugged into the far-end device 3 or integrated into the far-enddevice 3 as shown in FIG. 1A. As for the single ended operation as shownin the variants of FIGS. 1A, 1B, the span loss of the optical span 4 canbe calculated as follows:Power_(Rx(Near-End))=Power_(Tx(Near-End))−(2x(Estimated Span Loss)+10log 0.0x) in dB,

wherein Power_(Rx(Near-End)) is the signal power of the optical pilotsignal received by the transceiver 1,

Power_(Tx(Near-End)) the signal power of the optical pilot signaltransmitted by the transceiver 1,

estimated span loss is the determined span loss of the optical span 4,and

x the percentage of power looped back by loop back unit 5 or the loopback adapter 6 to the transceiver 1.

The span loss of the optical span 4 is therefore given by:

${{Estimated}\mspace{14mu}{Span}\mspace{14mu}{Loss}} = {\frac{{Power}_{{Tx}{({{Near} - {End}})}} - {Power}_{{Rx}{({{Near} - {End}})}} - {10\;\log\; 0.0\; x}}{2}\mspace{14mu}{in}\mspace{14mu}{dB}}$

With a predetermined desired receive power at the far-end device 3, e.g.the desired power received at the x % loop back unit, an attenuation ofa variable optical attenuator integrated in the transceiver 1 can becalculated as follows:Attenuation_(voa)=Power_(Tx(Near-End))−EstimatedSpanLoss−Power_(Rx(Far-End-Desired))in dB

The single ended operation for performing an automatic power adjustmentin an optical transceiver 1 according to the present invention iscorrect if the span losses for both fibers 4-1, 4-2 between the near endand the far-end devices are the same. This is true for mostapplications, because the optical fibers 4-1, 4-2 are arranged withinthe same strand of optical fibers and undergo the same externalinfluences.

FIG. 2 shows a diagram for illustrating a further possible embodiment ofa method and apparatus for performing an automatic power adjustmentaccording to the present invention. In the embodiment shown in FIG. 2 acommunication channel is used for power adjustment. In the embodimentshown in FIG. 1, the devices 2, 3, i.e. the near end device 2 as well asthe far-end device 3, comprise an optical transceiver 1 according to thepresent invention. The optical transceivers 1-1 and 1-2 can be pluggabletransceivers plugged into the respective devices 2, 3. In thisembodiment, the receiver calculates the difference between the receivedpower and the desired received power and sends a message to the othertransceiver to adjust the transmit power level by that amount. In theembodiment shown in FIG. 2, a communication channel can be establishedvia the optical span 4 between both pluggable optical transceivers 1-1,1-2. A communication channel can be formed by an embedded communicationchannel ECC or by an out of band communication channel. In theembodiment shown in FIG. 2, the optical transceiver 1-1 can perform thefollowing steps. In a first step, the signal power level of an opticalsignal received by the optical transceiver 1-1 from the other opticaltransceiver 1-2 via the optical span 4 is measured an integratedmeasuring unit of the near end optical transceiver 1-1. In a furtherstep, the measured power level is compared with a predetermined desiredreceive power level to calculate a difference between the measuredreceived power level and the desired received power level which can beperformed by a calculation unit integrated in the near end opticaltransceiver 1-1. In a further step, the near end optical transceiver 1-1transmits a power adjustment message via the established communicationchannel to the other optical transceiver 1-2 to adjust an attenuation oran amplification of the signal power of the optical signal transmittedby the far end optical transceiver 1-2 via the optical span 4 to thenear end optical transceiver 1-1. The power adjustment message can betransmitted, in a possible embodiment, periodically by the opticaltransceiver via the established communication channel to the otheroptical transceiver.

An advantage of the dual ended operation for performing power adjustmentas shown in FIG. 2 over the single ended operation as shown inconnection with FIGS. 1A, 1B is that in band cross-talk can be avoided.For the single ended operation, as shown in FIGS. 1A, 1B, a fraction ofthe signal being looped back at the far-end device 2 towards thetransceiver 1 of the near end device 2 may constitute in band crosstalkfor the transceiver 1 of the far end device 3 in the direction towardsthe transceiver 1 of the near end device 2. The magnitude of thiscross-talk depends on the power levels of the transmitters on both endsof the link, on the span loss in both directions, and on the fraction ofthe signal being looped back through a splitter/coupler combination ofthe far-end device. A high cross-talk value can degrade a signalperformance. On the other hand, reducing the amount of looped backsignal through selection of a higher ratio of the splitter/coupler paircan reduce the signal power of the optical pilot signal below anacceptable limit for a reliable operation. Accordingly, in theembodiment shown in FIGS. 1A, 1B, an optimum signal power for both endsof the link is required to achieve a low cross-talk penalty and a validoptical pilot signal at the same time. This is difficult to achieve withfixed power transmitters and the use of external attenuators. With theoptical transceiver, according to the present invention, having anintegrated signal adjustment unit such as a variable optical attenuatoror a variable gain optical amplifier, a desired balance between lowcross-talk and sufficient optical pilot tone signal power can beachieved. An advantage of the embodiment shown in FIGS. 1A, 1B residesin the fact that no separate communication channel between the opticaltransceivers has to be established.

In both embodiments in FIGS. 1A, 1B (Single-Ended-Operation) and FIG. 2(Dual-Ended Operation), the signal power level of the optical signal isadjusted automatically such that it is balanced with signal power levelsof other optical signals transmitted over the same optical span 4 atdifferent wavelengths. Furthermore, the signal power level of an opticalsignal transmitted by the optical transceiver 1 can be adjustedautomatically in real time to compensate changes of the transmissioncapability of the optical span 4. For example, if optical fibers of theoptical span 4 are bent, the transmission capability of the span 4 isreduced. With the method and transceiver according to the presentinvention, the signal power level of the optical signal transmitted bythe optical transceiver 1 is adjusted automatically in real time tocompensate for the reduced transmission capability of the optical span 4caused by bending the fibers.

In an optical network, it is desired to provide a flat optical spectrum.This is explained in connection with FIG. 10. The optical transceiver 1,according to the present invention, adapts its transmit power levelsuntil a far-end transceiver receives a flat optimal power levelspectrum. In a WDM optical network, there are transmitted signals viadifferent optical channels each having a different wavelength 2. As canbe seen in FIG. 10A, there can be different optical channelstransmitting optical signals with different power levels. Some channelshave a power level which is beyond a desired receive power level andother optical channels transmit optical signals having signal levelswhich do not achieve the desired receive power level at the receivingtransceiver. With the method and transceiver according to the presentinvention, the near end transceiver adjusts its transmit power levelsuntil a far-end transceiver receives the optimal power level as shown inFIG. 10B. Having all the wavelengths of the optical spectrum at the samepower level helps to keep the power levels within acceptable limits andalso helps to prevent signal degradation caused by the followingimpairments which can impact the signal performance with improper, powerlevels at the receiver side. If the signal power level is low, thereceiver noise is too high at the receiver. If the signal power is high,the receiver may saturate or become overpowered. If the signal power islow at the input of an optical amplifier, the optical signal noise ratioOSNR is low as the amplifier contributes amplified spontaneous emissionASE. If the signal power of the signal launched into a fiber span 4 ishigh, the optical signal can be distorted due to fiber non-lineareffects. Similar effects can occur with high signal power levels, forexample in Erbium doped fiber amplifiers or dispersion compensationfiber. A large difference in wavelengths powers between adjacentwavelengths λ within an optical spectrum furthermore can generatecross-talk from the high power wavelength λ_(H) to a low powerwavelength λ_(h). This kind of crosstalk can occur in optical filters oroptical demultiplexing equipment. For a single ended optical transceiverwhich utilizes a percentage of the signal looped back towards thetransceiver as shown in FIGS. 1A, 1B, a high level signal power canresult in high levels of cross-talk since the percentage of the signalbeing transmitted from the near end transceiver is combined on thereturn signal path with the signal being transmitted from the far-end,i.e. the percentage of the signal transmitted from the near endtransceiver becomes cross-talk for the far-end transmitted signal. Afurther advantage of having a flat signal spectrum having all wavelengthequalized is that this helps with trouble shooting optical networks as awavelength power level that is too low or is too high can be easilyidentified for an equalized spectrum versus an unequalized spectrum whenviewing or monitoring the entire optical spectrum. Accordingly, with themethod and apparatus according to the present invention, the automaticpower adjustment is performed in a preferred embodiment for each opticalchannel or wavelength λ to achieve a flat receive signal spectrum at thefar-end device.

FIG. 3 shows the use of an optical transceiver 1 according to thepresent invention wherein the optical transceiver 1 is placed in arouter device. In this embodiment, the optical transceiver 1 is used ina point-to-point connection between a far-end device 3 and a near enddevice 2. The near-end device 3 can be, for example, a router module ofa router 8. The far-end device 3 in the shown example can be atransponder or muxponder of an optical add/drop multiplexing node 9. Theoptical add/drop multiplexing node 9 can form a node of an opticalnetwork such as a DWDM optical network.

FIG. 4 shows a diagram of an example for an optical network 10. In theshown embodiment, the optical network 10 is a node ring network whereinseveral nodes 1-8 are connected to each other in a ring structure andcan perform an add/drop function for adding and dropping channelsto/from devices connected to the respective node 1-8. In a possibleembodiment, the nodes 9-1, 9-2, 9-3, 9-4 are connected with each otherover long distance optical fibers wherein each optical node 9-icomprises a network east interface and a network west interface at bothsides for connecting to the next two node 9-(i−1), 9-(i+1) of theoptical network 10. Each network interface can be connected via acorresponding optical fiber pair to a next node. In a possibleembodiment as shown in FIG. 4, the optical network 10 comprises linksbetween the nodes 9-i, each link comprising two optical fibers, whereineach optical fiber pair connects a network west interface of an opticalnode with a network east interface of the next optical node within thering. The optical signal received by a node 8-i within the opticalnetwork can either be dropped at the respective node or pass through tothe next node within the ring. An optical signal generated by a deviceconnected to the optical node 9-i can be added to the network 10 bymeans of the node 9-i.

FIG. 5 shows a block diagram of a possible embodiment of an add/dropmultiplexing node 9-i for an optical network 10 according to the presentinvention.

As can be seen in FIG. 5, the optical network node 9-i comprises a firstnetwork interface 11 and a second network interface 12. Both networkinterfaces 11, 12 are bidirectional interfaces and connect the networknode 9-i to the adjacent network nodes 9-(i−1) and 9-(i+1) within theoptical network 10. The network interfaces 11, 12 are connected to acorresponding multichannel multiplexing/demultiplexing unit 13, 14comprising a predetermined number of channel ports. In the embodimentshown in FIG. 5, both multichannel multiplexing/demultiplexing unitscomprise forty channel ports. Both multichannelmultiplexing/demultiplexing units 13, 14 comprise an internal opticalloop 15, 16 which can reflect a small percentage of an optical signalback towards an optical transceiver. Furthermore, both optical networkinterfaces 11, 12 can comprise an attenuator 17, 18. An external fixedattenuator 17, 18 may be installed to adjust pass-through wavelengthpower levels to locally added wavelengths power levels. In the exampleshown in FIG. 5, both ports 39 of the multichannelmultiplexing/demultiplexing unit 13, 14 are connected with each other toform a pass through optical channel. By means of the attenuators 17, 18it is possible to adjust the power level of the pass through wavelengthof this channel to locally added wavelength power levels. In theexemplary embodiment in FIG. 5, some channel ports of themultiplexing/demultiplexing units 13, 14 are connected to transpondersor muxponders of the optical node 9-i. In the embodiment shown in FIG.5, the first channel port of the first multichannelmultiplexing/demultiplexing unit 13 is connected to a transponder 19provided for a single client device 20 connected to the network node9-i. The client device 20 can for example be a router or a switch.Furthermore, in the exemplary embodiment of FIG. 5, the second channelof the first multichannel multiplexing/demultiplexing unit 13 isconnected to a muxponder 21 provided for multiple client devices 22-1 to22-N. In the exemplary embodiment, the last channel 40 of the firstmultiplexing/demultiplexing unit 13 is again connected to anothertransponder 19 of a further single client device 20. In the exemplaryembodiment, the channels 1, 2 of the second multiplexing/demultiplexingunit 14 are connected by means of transponders 19 to single clientdevices 20. In the exemplary embodiment, the last channel 40 of thesecond multiplexing/demultiplexing unit 14 is connected via a muxponder21 to client devices 22-1 to 22-N performing a group of client devices.In the example shown in FIG. 5, each multiplexing/demultiplexing unit13, 14 comprises 40 channel ports. In alternative embodiments, thenumber of channel ports can be higher or lower. In the embodiment shownin FIG. 5, each muxponder 21 is connected to N=4 client devices. In analternative embodiment, the number of client devices connectable to onemuxponder 21 can vary.

As can be seen in FIG. 5 the muxponder 21 and the transponders 19 cancomprise optical transceivers 1 according to the present invention. Theoptical transceivers 1 are connected to a channel port of amultiplexing/demultiplexing unit 13, 14. The optical transceiver 1 in apossible embodiment can be plugged into a corresponding transponder 19or muxponder 21. The optical transceivers 1 comprise each a signaladjustment unit adapted to adjust automatically a signal power level ofan optical signal transmitted by the respective optical transceiver 1via an optical span towards the respective channel port of themultichannel multiplexing/demultiplexing unit 13, 14. In the exemplaryembodiment shown in FIG. 5, the transmitting optical transceivers 1comprise a monitoring unit for monitoring at least one optical pilotsignal transmitted by the optical transceiver via a first fiber towardsthe multichannel multiplexing/demultiplexing unit 13, 14 and a loopedback via a second fiber to the optical transceiver 1. In a possibleembodiment, the monitoring unit measures the received signal power levelof the looped back optical pilot signal and determines the span loss ofthe optical span on the basis of the transmitted signal power of thetransmitted optical pilot signal and on the basis of the measuredreceived signal power level of the received looped back optical pilotsignal. In the embodiment shown in FIG. 5, the optical pilot signaltransmitted by a transceiver is looped back by means of loop back units15, 16 to reflect a small percentage of the optical signal back towardsto the transmitting optical transceiver 1. In a possible alternateembodiment, the automatic adjustment of the signal power level isperformed in response to a determined span loss of at least one opticalspan connecting the transceiver with a receiving transceiver.

An optical span can be formed by the optical connection between atransceiver 1 and the respective channel port of themultiplexing/demultiplexing unit 13, 14 (that may not contain the looptowards the transceiver) and a long distance optical span to atransceiver located in another node of the optical network. Accordingly,each optical transceiver 1 thus automatically adjusts a signal powerlevel of an optical signal transmitted by the optical transceiver 1towards the respective channel port corresponding to a determined spanloss to achieve a predetermined desired receive signal power level ofthe optical signal received by a receiving transceiver within the sameor another node. Consequently, different receiving transceivers receivethe flat optical power spectrum as shown in FIG. 105. Since eachtransponder and muxponder is connected to a multiplexer/demultiplexer,all other channels are filtered out. The flat spectrum is actuallyexperienced on the fiber spans between nodes.

In a further embodiment as shown in FIG. 5, the optical transceivers 1connecting the transponders 19 and the muxponders 21 to the channels ofthe multichannel multiplexing/demultiplexing units 13, 14 are adapted tooperate in a single-ended mode as explained in more detail withreference to the embodiments shown in FIGS. 1A, 1B.

Each transponder 19 comprises further an optical transceiver 1′connecting the transponder 19 to a client device 20 such as a router ora switch. The client device 20 can comprise a plugged-in opticaltransceiver 1″ being connected to the optical transceiver 1′ of thetransponder 19 via optical fibers. The client device 20 can also beconnected directly by means of an optical transceiver 1″ to amultiplexing/demultiplexing unit 13, 14 without going through atransponder or muxponder as illustrated also in FIG. 5. The opticaltransceivers 1′, 1″ can perform an automatic power adjustment accordingto the present invention. The optical transceiver 1′ and 1″ can operatein the shown embodiment in a dual-ended mode as explained in moredetailed with reference to the embodiment shown in FIG. 2.

FIG. 6 shows a diagram for illustrating a dual-ended operation over anoptical network using the method and transceiver for performing anautomatic power adjustment according to the present invention.

In a first step, the transceiver 1 plugged into a transponder 19 of afirst optical network node 9-1 turns on a light source such as a laser.The optical transceiver 1 transmits an optical signal viamultiplexing/demultiplexing units 14 and the optical network 10 to areceiving optical network node 9-2 comprising also a transponder 19 witha plugged-in optical transceiver 1 for operating in a dual-ended node.The transceiver 1 within the receiving optical node 9-2 receives theoptical signal and measures the power level of the received opticalsignal. The receiving optical transceiver 1 within the receiving node9-2 calculates the difference between the received signal power leveland a predetermined desired receive power level. The receiving opticaltransceiver 1 within the receiving node 9-2 then sends a message to thetransmitting optical transceiver 1 within the transmitting node 9-1indicating the required power adjustment over an establishedcommunication channel. The transceiver 1 within a transponder 19 of thefirst optical network node 9-1 receives message from the far-end opticalnetwork node 9-2 via the established communication channel. Then, asignal adjustment unit integrated in the sending optical transceiver 1within the first optical node 9-1 adjusts automatically the transmittedpower to optimize the far-end receive power level. This signal poweradjustment can be performed by means of a variable optical attenuator(VOA) or by means of a variable gain optical amplifier. The receivingoptical transceiver 1 within the second optical network 9-2 measures, ina further step, the received optical power level which corresponds tothe desired optimal power level.

FIG. 7 shows a block diagram of a possible embodiment of a transponder19 comprising a pluggable long-reached transceiver 1 and a pluggableshort-reached transceiver 1′. In the shown embodiment the pluggablelong-reached transceiver 1 comprises a signal adjustment unit 23 beingformed by a variable optical attenuator. The variable optical attenuator23 is adapted to adjust automatically a signal power level of an opticalsignal transmitted by said pluggable long-reached transceiver 1 via oneor several optical spans to a far-end device which can be integrated ina far-end optical node of the optical network 10. This power leveladjustment is performed in response to a determined span loss of one orseveral optical spans connecting the transmitting transceiver 1 andremote transceiver to achieve a predetermined desired receive signalpower level of the optical signal received by the remote transceiver inthe far-end device. The pluggable long-reach transceiver 1 further cancomprise a DWDM transmitter 24 controlled by an in band communicationchannel control device 25. The pluggable long-reach transceiver 1further comprises a receiver 26 to receive an optical signal from theremote transceiver of the far-end device. The communication channel canbe formed by an embedded communication channel ECC or by an out of bandcommunication channel. The communication channel is provided fortransmitting a power adjustment message to the other remote opticaltransceiver and to receive a power adjustment message from the remoteoptical transceiver. This power adjustment message is received by thereceiver 26 via the established communication channel and is extractedby the in band communication channel control unit 25 to be processed bya processor 27 of the transponder 19. In response to the received poweradjustment message, the attenuation of the signal power of the opticaldata signal transmitted by the optical pluggable long-reach transceiver1 is adjusted. In a possible embodiment, the processor 27 controls thevariable optical attenuator 23 depending on the received poweradjustment message.

The transponder 19 as shown in FIG. 7 can comprise a further pluggableshort-reach transceiver 1′ for connecting a client device 20 with thetransponder 19. The pluggable short-reach transceiver 1′ comprises areceiver 28 and a short-reach transmitter 29.

The transponder 19 can further comprise further data processing unitsfor performing a processing of protocol frames or for performancemonitoring and forward error correction. These processing units can beintegrated into a processing unit 32 as shown in FIG. 7. For both signalpaths of the transponder 19, a clock recovery circuit 30, 31 can beprovided.

FIG. 8 shows a further possible embodiment of a transponder 19comprising a pluggable long-reach transceiver 1. In the shownembodiment, the pluggable long-reach transceiver 1 comprises a variablegain optical amplifier 33 instead of a variable optical attenuator.

FIG. 9 shows a block diagram of a further possible embodiment of anoptical transponder 19. In the shown embodiment, the long-reachtransceiver is not in the form of a plugged-in device but is integratedin the transponder 19. The optical transponder 19 comprises a long-reachinterface consisting of discrete transmitter and receiver components anda variable optical attenuator 23. In the shown embodiment of FIG. 9, theshort-reach transceiver 1′ of the transponder 19 still forms a pluggabledevice.

What is claimed is:
 1. A method for performing an automatic poweradjustment, wherein a signal power level of an optical signaltransmitted by an optical transceiver via at least one optical span to afar-end device is adjusted automatically in response to a determinedspan loss of said optical span to achieve a predetermined desiredreceive signal power level of the optical signal at the far-end-device;and wherein at least one optical pilot signal is transmitted by saidoptical transceiver via a first fibre of said optical span towards thefar-end device and looped via a second fibre of said optical span backto said optical transceiver which measures the signal power level of thelooped back optical pilot signal received by said optical transceiver.2. The method according to claim 1, wherein the span loss of saidoptical span is determined on the basis of a transmit signal power levelof the transmitted optical pilot signal and on the basis of the measuredreceive signal power level of the received looped back optical pilotsignal.
 3. The method according to claim 2, wherein the optical pilotsignal is looped back by a signal loop back unit being formed by a loopadapter connected to said optical span at the side of the far-end-deviceor plugged into the far-end-device.
 4. An optical transceiver comprisinga signal adjustment unit adapted to adjust automatically a signal powerlevel of an optical signal transmitted by said optical transceiver viaat least one optical span to a far-end-device in response to adetermined span loss to achieve a predetermined desired receive signalpower level of the optical signal received by said far-end-device,wherein a monitoring unit is provided for monitoring at least oneoptical pilot signal transmitted by said optical transceiver via a firstfibre of said optical span towards the far-end-device and looped backvia a second fibre of said optical span to said optical transceiver,wherein said monitoring unit measures the received signal power level ofthe looped-back optical pilot signal and determines the span loss on thebasis of the transmitted signal power of the transmitted optical pilotsignal and on the basis of the measured received signal power level ofthe received looped back optical pilot signal.
 5. An add/dropmultiplexing node for an optical network comprising: at least onenetwork interface connected to a multichannelmultiplexing/demultiplexing unit comprising a predetermined number ofchannel ports each being connected to an optical transceiver having asignal adjustment unit to adjust automatically a signal power level ofan optical signal transmitted by said optical transceiver towards therespective channel port to provide a predetermined receive signal powerlevel of the optical signal, wherein the multichannelmultiplexing/demultiplexing unit comprises a loop back unit to loop anoptical pilot signal transmitted by said optical transceiver towardssaid channel port of said multiplexing/demultiplexing unit back to theoptical transceiver, and wherein the optical transceiver determines aspan loss on the basis of the transmitted signal power level of thetransmitted optical pilot signal and on the basis of the looped-backoptical pilot signal.